Condenser bushing

ABSTRACT

The present disclosure relates to a condenser bushing including a condenser core and electrically conductive field-grading layers, which are embedded in insulating material of the condenser core and arranged around a central channel for conductor extending along an axis defining an axial direction, while an electric connection is provided to at least one of the field-grading layers, wherein pairs of neighbouring field-grading layers with the insulation material between them form sections of the condenser core of axial lengths L 1  through L n  and with capacitances C 1  through C n , characterized in that a shape of at least one of the field-grading layers deviates from cylindricality in order to reduce non-uniformity of electric field stress of the condenser bushing compared to a corresponding condenser bushing with the cylindrical field-grading layers forming sections of the axial lengths L 1  through L n  and with capacitances C 1  through C n .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 national stage application of PCTInternational Application No. PCT/EP2020/087675 filed on Dec. 22, 2020,which in turn claims foreign priority to European Patent Application No.19220097.0, filed on Dec. 30, 2019, the disclosures and content of whichare incorporated by reference herein in their entirety.

TECHNICAL FIELD

The subject of the disclosure is a condenser bushing applicable inelectric power engineering.

BACKGROUND ART

A high voltage bushing is a component that is mainly used to carrycurrent at high electric potential from an active part of a first highvoltage component, such as a transformer, a generator or a circuitbreaker, through a barrier, like the grounded housing of the firstcomponent, to a second high voltage component, such as a high voltageoverhead line or a high voltage cable termination. Such a high voltagebushing is used in switchgear installations (such as gas-insulatedswitchgear, GIS or air-insulated switchgear, AIS), power or distributiontransformers, or in high voltage rotating machines like generators, forvoltage levels ranging from a few kV up to several hundred kV and above1000 kV. In order to decrease and control the electric field, the highvoltage bushing comprises a condenser core, which facilitateselectric-field stress control. Condenser bushing can also be used as apart of a high-voltage insulation system of an instrument transformer ora cable termination.

A typical condenser bushing for medium- or high-voltage applications, asshown in FIG. 1 , comprises a condenser core (1) with a number ofconcentric electrically conducting field-grading layers (3) ofcylindrical shape arranged around the central conductor (2) embedded inthe insulation material of the condenser core (1). The innermostfield-grading layer of the condenser core of a bushing is electricallyconnected to the conductor of the bushing via a high-voltage connection(5). The outermost, and/or one of the other outer field-grading layersare electrically connected to the ground potential via a groundconnection (6). Connection to the ground potential goes typically viathe metallic flange (4) which serves to mechanically fix the bushing tothe grounded equipment. The field-grading layers (3) with the insulationmaterial between them form a capacitive divider distributing the totalvoltage U applied to the condenser core among the field-grading layersin fixed and defined portions U. Each pair of neighbouring field-gradinglayers (3) with the insulation material between them forms a section ofthe condenser core with a capacitance C_(i). Each voltage portion U_(i)is proportional to the inverse of the section capacitance, 1/C_(i), andthe sum of the portions add to the total voltage U. As a result, theelectric field generated by the high voltage is distributed in acontrolled way, both inside the condenser core in the radial direction(radial field stress), and outside, close to the outer surface of thebushing, along its axis (axial field stress). The condenser core shownin FIG. 1 has four sections, in general an arbitrary number n ofsections can be applied.

Condenser bushings of today are usually manufactured by winding a spacermaterial on a mandrel or conductor and inserting electrically conductivefield-grading layers (foils) in between the wound layers of the spacermaterial. Alternatively, the field-grading layers are printed using anelectrically conductive ink directly on the surface of the spacermaterial. Afterwards the structure is impregnated by hardenable resinwhich is subsequently hardened. This method allows for producingfield-grading layers of cylindrical shape only. However additivemanufacturing methods allow to improve the features of the condenserbushings.

One problem to be solved is to lower the diameter of a condenserbushing.

Another problem are potential connections of the field-grading layers,each of which consists of a piece of conductor glued to the layer usingan electrically conductive adhesive. The construction of such aconnection is complex, prone to errors in manufacturing and makes arelatively high inductance.

Still another problem is an electric field enhancement at the surface ofthe condenser core due to the sharp edges of the field grading layerswhich are close to the surface. To minimize this enhancement there is aneed of a thick overbuild layer of the insulating material covering theedges of the field-grading layers all over the axial distance adjacentto the edges. This overbuild thickness has to be even larger because ofthe typical wide tolerance of both radial and axial positions of thefoils in the manufacturing process of the condenser core.

SUMMARY

A condenser bushing with cylindrical field-grading layers (foils) isshown in FIG. 1 . In order to generate possibly uniform grading of theelectric field in both the axial and the radial directions the layerspacing δ₁′ through δ_(n)′ of all the sections and the axial lengths ofthe sections L₁ through L_(n) have to be adjusted accordingly. Forexample, they are adjusted so that the capacitances of the thus formedsections of the condenser core, C₁ through C_(n), are equal. This leadsto an equal voltage division through the sections of the condenser coreand, when the axial lengths of the sections are linearly distributed, toa substantially uniform axial field stress of all sections. Then,however the layer spacing distances δ_(i)′ are typically not equal, andso are the field stress values in the radial direction between thefoils, which are the ratios of the section voltage U_(i) and the layerspacing distance δ_(i)′. This implies that only one or two sections canbe stressed to a maximal allowable value. The other sections areunder-stressed, which means that the diameter of the condenser core islarger than a possible optimal value. This issue concentrates at thesharp edges at the axial ends of the layers, which locally enhance thefield stress and make the field in each single section highlynon-uniform over the distance between the edge of the foil and theneighbouring foil of the section. Because of this non-uniformity theregion of the edge of the foil is the most prone to electric breakdownbetween the layers. To avoid such a breakdown the condenser core has tobe designed so that, for each section, the ratio of the voltage U_(i) ofthe section and the radial width of the section at its end, i.e. at theedge of the axially shorter layer of the section, is smaller than adefined safe design limit value. This ratio will be referred to as amean edge field stress throughout the description of the disclosure. Fora condenser core with cylindrical layers the radial section width at thelayer edge, i.e. at the axial end of the section, is equal to the layerspacing δ_(i)′ but generally it does not need to be so and throughoutthe description of the disclosure we shall use the designation δ_(i)′ orδ_(i) to denote the radial width of the section at its axial end.Generally, this width can be different at both axial ends of thesection; in the description we discuss one end only, but the disclosurerelates equivalently to the second end. The safe design value of themean edge field stress is strongly limited due to the sharp edges of thelayers, this imposes designs with relatively large δ_(i)′, and thus withrelatively large overall diameter of the bushing.

Use of the additive manufacturing techniques, in which the insulatingmaterial forming the condenser core and the conductive material formingthe field-grading layers can be deposited in a controlled way overdefined locations of the build-up surface of the manufactured condensercore, layer by layer, allows to solve the problems of not equalstressing of all sections and of the strong limitation of the mean edgefield stress by providing a condenser core of a bushing withnon-cylindrical, curved field-grading layers. In this way, additivemanufacturing allows for optimization of the field stress distributionin a condenser core leading to a possibility of reducing its diameterand consequently for reduction in material usage, processing time, andcost of the component.

In one example the field-grading layers are shaped so that thedifferences between the mean edge field stress values of the sectionsare reduced, or that all the mean edge field stress values are madeequal. For example, one of the field-grading layers, forming a sectionwhich in an optimized design with cylindrical layers is stressed themost, reaching the safe design limit value, is shaped so that the layerspacing of the section is maintained over the majority of the layersurface area but in the regions close to the edge of the second of thelayers forming the section the distance between the layers is increased.In this way the voltage of the section is substantially unchanged, butthe mean edge field stress of that section is decreased. At the sametime the mean edge field stress of an adjacent section is increased, butso that it does not reach the safe design limit. This change allows forproportionally decreasing of all the layer spacing distances of thecondenser core and thus reducing its overall diameter, until the maximalmean edge field stress reaches again the safe design limit value.Optimization of all the layers in such a manner can also lead to makingthe mean edge field stress substantially equal for all sections, thusallowing for a significant reduction of the overall diameter of thecondenser core.

In another example the edges of the field-grading layers are bentoutwards so that the concentration of the electric field is reduced atthe axial ends of the section and the field stress value over the pathbetween the layers in the vicinity of the edges is made more uniform.With the more uniform field stress the breakdown voltage of the sectionbecomes larger and thus also the safe design limit of the mean edgefield stress value can be set at a higher point. This allows for asignificant reduction of the layer spacing distance and thus for areduction of the overall diameter of the condenser core.

Additionally, the additive manufacturing techniques can providepotential connections being an integral part of the field-gradinglayers, made in substantially axially symmetric shape, with theconductive material volume reaching from the layer to the outer or tothe inner surface of the condenser core. This simplifies theconstruction of the connection in that a smaller number of components isused and in that it does not require additional manufacturing procedurescompared to those used to produce the insulation and the field-gradinglayers of the core. This makes also the inductance of the connectionssignificantly smaller compared to a connection made at one point with apiece of wire.

Moreover, use of the additive manufacturing techniques allows to shapean outer surface of the condenser core in such a way that the insulatingmaterial overbuild thickness over the edges of the field-grading layersis made larger than in the sections between the edges. Contrary to thespacer-winding-impregnation-and-curing manufacturing techniques oftoday, the additive manufacturing allows for precise synchronization ofthe positions of the edges of the layers and of the protruding parts ofthe outer shape of the condenser core. Thus, the problem of an electricfield enhancement at the surface of the condenser core can be solvedwith using a minimum amount of the insulating material, which is appliedonly there where it is needed to reduce the field at the surface.

The present disclosure relates to a condenser bushing comprising acondenser core (1) and electrically conductive field-grading layers (3),which are embedded in insulating material of the condenser core (1) andarranged around a central channel for conductor (2) extending along anaxis defining an axial direction, while an electric connection (6) isprovided to at least one of the field-grading layers (3), wherein pairsof neighbouring field-grading layers (3) with the insulation materialbetween them form sections of the condenser core of axial lengths L₁through L_(n) and with capacitances C₁ through C_(n), wherein a shape ofat least one of the field-grading layers (3) deviates fromcylindricality in order to reduce non-uniformity of electric fieldstress of the condenser bushing compared to a corresponding condenserbushing with the cylindrical field-grading layers forming sections ofthe axial lengths L₁ through L_(n) and with capacitances C₁ throughC_(n), wherein at least one of the field-grading layers (3) is shapedsuch that the diameter of said field-grading layer (3) varies along theaxial direction, characterized in that the diameter of said fieldgrading layer (3) has at least one maximum between the edges of thefield-grading layer (3).

The condenser bushing may further comprise any of below features ortheir technically feasible combinations:

-   -   at least one of the field-grading layers (3) is shaped such that        the diameter of said field-grading layer varies along the axial        direction;    -   the diameter of the field-grading layer (3) has at least one        maximum between the edges of the field-grading layer (3);    -   the mean edge field stress level, defined as the ratio of the        voltage U_(i) of the section and the radial width δ_(i) of the        section at its end, i.e. at the edge of the axially shorter        field-grading layer of the section, in at least one section        formed by a non-cylindrical field-grading layer is smaller than        in the corresponding section of a condenser bushing with        cylindrical field-grading layers forming sections of identical        capacitances C₁ through C_(n) and identical axial lengths L₁        through L_(n);    -   the absolute value of

$\left( {{\frac{U_{i}}{\delta i}/\frac{U_{j}}{\delta j}} - 1} \right)$

is at least 20% smaller than the absolute value of

$\left( {{\frac{U_{i}}{\delta_{i}^{\prime}}/\frac{U_{j}}{\delta_{j}^{\prime}}} - 1} \right),$

where

$\frac{U_{i}}{\delta i}{and}\frac{U_{j}}{\delta j}$

are the mean edge field stress levels of two neighbouring sections,wherein at least one section is formed by a non-cylindricalfield-grading layer and

$\frac{U_{i}}{\delta_{i}^{\prime}}{and}\frac{U_{j}}{\delta_{j}^{\prime}}$

are the mean edge field stress levels of two corresponding neighbouringsections the corresponding condenser bushing with the cylindricalfield-grading layers;

-   -   the radial widths of the sections at their axial ends are        substantially equal;    -   the innermost and/or the outermost field-grading layer is        cylindrical;    -   the capacitances of all the sections formed by the field-grading        layers (3) are equal;    -   at least one edge of at least one of the field-grading layers        (3) is bent outwards with respect to the axis;    -   the radius of curvature of the bent edge of the field-grading        layer is equal to at least three, preferably at least five,        layer thicknesses;    -   at least one potential connection (5, 6, 7) is an integral part        of a field-grading layer (3) and has a substantially axially        symmetric shape, with the conductive material volume reaching        from the field-grading layer to the outer or inner surface of        the condenser core (1);    -   the condenser core (1) is shaped in such a way that the        thickness of an insulating material between each of the edges of        adjacent field-grading layers (3) and the outer surface of the        condenser core (1) is greater than the thickness of an        insulating material between the middle point between the edges        of the adjacent field-grading layers (3) and the outer surface        of the condenser core (1).

The present disclosure relates also to a use of an additivemanufacturing method to manufacture the condenser bushing.

BRIEF DESCRIPTION OF DRAWINGS

Condenser bushing is depicted in exemplary embodiments, wherein figurespresent in a cross section:

FIG. 1 —prior art condenser bushing,

FIG. 2 —first embodiment,

FIG. 3 —second embodiment,

FIGS. 4 and 5 —condenser bushing comprising potential connections,

FIG. 6 —condenser bushing comprising insulation material surfacefollowing field-grading layer edges

DETAILED DESCRIPTION

Manufacturing of a bushing using additive manufacturing methods allowsfor manufacturing the field-grading layers (3) of an arbitrary shape. Anexample of such a bushing is shown in FIG. 2 . In this embodiment thefield-grading layers are shaped so that the radial width of all sectionsat their ends, δ_(i), are equal. The innermost and the outermost layersare cylindrical. The other layers are shaped so that the capacitances ofall the sections are also equal. This makes the mean edge field stressvalues of all the sections equal, all reaching the safe design limit,and allows for making the overall diameter of the condenser coresignificantly smaller than in an equivalent design with cylindricallayers in which the mean edge field stress value reaches the safe designlimit only in one or two sections.

In the design shown in the drawing, the equivalent grading system formedby cylindrical field-grading layers, having all sections of the samecapacitances C₁ through C₄ and the same axial lengths of the layers L₁through L₄, the section C₄ would be the only one with the mean edgefield stress level reaching the safe design limit. In the gradingsystem, by non-cylindrical shaping of the inner layer of the section C₄,the edge width δ₄ of that section is increased compared to thecylindrical design. In that way, the mean edge field stress level ofthis section is reduced and the radial dimension of the set of alllayers can be proportionally scaled down to a smaller diameter, bringingback the mean edge field stress value of the section C₄ to the safedesign limit. In such a way the diameter of the condenser core can bemade smaller than that of the one made according to known art. Thediameter of the field-grading layer (3) has at least one maximum betweenthe edges of the field-grading layer (3). Therefore, a capacitancebetween adjacent field-grading layers (3) can be altered by adjustingthe position, the width or the amplitude of the maximum of each of fieldgrading layer (3). In this way the distance between adjacentfield-grading layers, and thereby also the capacitance and the mean edgefield stress, can be adapted. As the maximum of the field-grading layer(3) reduces the distance of between adjacent field-grading layers (3) astronger electric field is stored at the maximum, hence, reducing theelectric field strength at the edges. In the embodiment shown in FIG. 2the maximums of the field-grading layers (3) have been designed suchthat the maximums of the field-grading layers (3) become bigger inamplitude, but narrower in width, with increasing distance from thecondenser core (1). In the example in FIG. 2 all the layers areoptimized in the described way, bringing the mean edge field stress toan equal value in all the sections and providing a significant reductionof the diameter compared to the design with cylindrical layers.

Another embodiment is shown in FIG. 3 . The edges of the field-gradinglayers are bent outwards thus reducing the electric field stress closeto the edges, making the field stress more uniform over the distancebetween the layers at the end of the section and allowing for settingthe safe design limit of the mean edge field stress level at a highervalue compared to a condenser core with cylindrical layers.

FIGS. 4 and 5 show the potential connections, high-voltage (5), ground(6), and voltage-tap (7), made in a form of an axially symmetric bulkconductive material objects, produced in an additive manufacturingprocess in parallel with the insulation material of the condenser core.The diameter of the inner field-grading layers (3) have a maximumbetween the edges of the field-grading layer (3). Hence, the electricfield is accumulated at the maximums and the electric field at the edgesof the field-grading layers (3) is reduced. The capacitance betweenfield-grading layers is adjusted and levelled by forming the maximumsaccordingly.

FIG. 6 shows the shape of the condenser core (1) with its outer surfacefollowing the edges of the field-grading layers (3). The insulationmaterial thickness is enlarged close to the edges of the layers. Thesurface of the condenser core (1) is stepped such that the thickness ofthe insulating material between the each of the edges of adjacentfield-grading layers (3) and the outer surface of the condenser core (1)is greater than the thickness of an insulating material between themiddle point between the edges of the field-grading layers (3) and theouter surface of the condenser core (1). Hence, the corner ofstep-shaped outer surface of the condenser core (1) is positioned at alevel between adjacent field-grading layers (3). In this way excessiveelectric fields between the edge of the field-grading layers (3) and theouter surface of the condenser core (1) are omitted. Both the insulationmaterial and the conductive layers can be made in an additivemanufacturing process allowing for a precise correlation of thepositions of the layers and the positions of the protruding parts of theinsulating material.

Potential connection (5, 6, 7) are suitable also for other types ofcondenser bushing, for example for a condenser bushing with acylindrical field-grading layers. Therefore a present disclosure relatesalso to a condenser bushing comprising a condenser core (1) andelectrically conductive field-grading layers (3) which are embedded ininsulating material of the condenser core (1) and arranged around acentral channel for conductor (2) extending along an axis defining anaxial direction, while an electric potential connection (6) is providedto at least one layer of the field-grading layers (3), the connectionbeing an integral part of a field-grading layer (3) and having asubstantially axially symmetric shape, with the conductive materialvolume reaching from the field-grading layer to the outer or innersurface of the condenser core (1).

The same applies to the outer surface following the edges of thefield-grading layers (3). The present disclosure relates also to acondenser bushing comprising a condenser core (1) and electricallyconductive field-grading layers (3) which are embedded in insulatingmaterial of the condenser core (1) and arranged around a central channelfor conductor (2) extending along an axis defining an axial direction,while an electric potential connection (6) is provided to at least onelayer of the field-grading layers (3), wherein the condenser core (1) isshaped in such a way that the thickness of an insulating materialbetween the edges of the field-grading layers (3) and the outer surfaceof the condenser core (1) is greater than the thickness of an insulatingmaterial between the section between the edges of the field-gradinglayers (3) and the outer surface of the condenser core (1).

REFERENCE NUMBERS LIST

1—condenser core

2—conductor

3—field-grading layers

4—flange

5—high-voltage connection

6—ground connection

7—voltage tap connection

8—curvature of field-grading layer

9—edges of field-grading layers bent outwards

10—curvature of outer surface of condenser core, where the insulationmaterial surface follows the field-grading layer edges

1. A condenser bushing comprising: a condenser core and electricallyconductive field-grading layers, which are embedded in insulatingmaterial of the condenser core and arranged around a central channel forconductor extending along an axis defining an axial direction, anelectric connection being provided to at least one of the field-gradinglayers, pairs of neighbouring field-grading layers with the insulationmaterial between them forming sections of the condenser core of axiallengths L₁ through L_(n) and with capacitances C₁ through C_(n), a shapeof at least one of the field-grading layers deviating fromcylindricality to reduce non-uniformity of electric field stress of thecondenser bushing compared to a corresponding condenser bushing with thecylindrical field-grading layers forming sections of the axial lengthsL₁ through L_(n) and with capacitances C₁ through C_(n), and at leastone of the field-grading layers being shaped such that the diameter ofsaid field-grading layer varies along the axial direction, the diameterof said field grading layer having at least one maximum between theedges of the field-grading layer.
 2. The condenser bushing according toclaim 1, wherein the mean edge field stress level, defined as the ratioof the voltage U_(i) of the section and the radial width δ_(i) of thesection at its end in at least one section formed by a non-cylindricalfield-grading layer is smaller than in the corresponding section of acondenser bushing with cylindrical field-grading layers forming sectionsof identical capacitances C₁ through C_(n) and identical axial lengthsL₁ through L_(n).
 3. The condenser bushing according to claim 2, whereinthe absolute value of$\left( {{\frac{U_{i}}{\delta i}/\frac{U_{j}}{\delta j}} - 1} \right)$is at least 20% smaller than the absolute value of$\left( {{\frac{U_{i}}{\delta_{i}^{\prime}}/\frac{U_{j}}{\delta_{j}^{\prime}}} - 1} \right),$where $\frac{U_{i}}{\delta i}{and}\frac{U_{j}}{\delta j}$ are the meanedge field stress levels of two neighbouring sections, wherein at leastone section is formed by a non-cylindrical field-grading layer and$\frac{U_{i}}{\delta_{i}^{\prime}}{and}\frac{U_{j}}{\delta_{j}^{\prime}}$are the mean edge field stress levels of two corresponding neighbouringsections the corresponding condenser bushing with the cylindricalfield-grading layers.
 4. The condenser bushing according to claim 1,wherein the radial widths of the sections at their axial ends aresubstantially equal.
 5. The condenser bushing according to claim 1,wherein the innermost and/or the outermost field-grading layer iscylindrical.
 6. The condenser bushing according to claim 1, wherein thecapacitances of all the sections formed by the field-grading layers areequal.
 7. The condenser bushing according to claim 1, wherein at leastone edge of at least one of the field-grading layers is bent outwardswith respect to the axis.
 8. The condenser bushing according to claim 7,wherein the radius of curvature of the bent edge of the field-gradinglayer is equal to at least three layer thicknesses.
 9. The condenserbushing according to claim 1, wherein at least one potential connectionis an integral part of a field-grading layer and has a substantiallyaxially symmetric shape, with the conductive material volume reachingfrom the field-grading layer to the outer or inner surface of thecondenser core.
 10. The condenser bushing according to claim 1, whereinthe condenser core is shaped in such a way that the thickness of aninsulating material between the each of the edges of adjacentfield-grading layers and the outer surface of the condenser core isgreater than the thickness of an insulating material between the middlepoint between the edges of the field-grading layers and the outersurface of the condenser core.
 11. An additive manufacturing method tomanufacture the condenser bushing according to claim
 1. 12. Thecondenser bushing according to claim 7, wherein the radius of curvatureof the bent edge of the field-grading layer is equal to at least fivelayer thicknesses.
 13. An electrical insulation system comprising: anactive part comprising a conductor; a condenser bushing disposed aroundthe conductor, the condenser bushing comprising: a condenser core andelectrically conductive field-grading layers, which are embedded ininsulating material of the condenser core and arranged around a centralchannel for conductor extending along an axis defining an axialdirection, an electric connection being provided to at least one of thefield-grading layers, pairs of neighbouring field-grading layers withthe insulation material between them forming sections of the condensercore of axial lengths L₁ through L_(n) and with capacitances C₁ throughC_(n), a shape of at least one of the field-grading layers deviatingfrom cylindricality to reduce non-uniformity of electric field stress ofthe condenser bushing compared to a corresponding condenser bushing withthe cylindrical field-grading layers forming sections of the axiallengths L₁ through L_(n) and with capacitances C₁ through C_(n), and atleast one of the field-grading layers being shaped such that thediameter of said field-grading layer varies along the axial direction,the diameter of said field grading layer having at least one maximumbetween the edges of the field-grading layer.
 14. The electricalinsulation system according to claim 13, wherein the active part is partof a high voltage component of one of a transformer, a generator, and acircuit breaker.
 15. The electrical insulation system according to claim13, wherein the mean edge field stress level, defined as the ratio ofthe voltage U_(i) of the section and the radial width δ_(i) of thesection at its end in at least one section formed by a non-cylindricalfield-grading layer is smaller than in the corresponding section of acondenser bushing with cylindrical field-grading layers forming sectionsof identical capacitances C₁ through C_(n) and identical axial lengthsL₁ through L_(n).
 16. The electrical insulation system according toclaim 14, wherein the absolute value of$\left( {{\frac{U_{i}}{\delta i}/\frac{U_{j}}{\delta j}} - 1} \right)$is at least 20% smaller than the absolute value of$\left( {{\frac{U_{i}}{\delta_{i}^{\prime}}/\frac{U_{j}}{\delta_{j}^{\prime}}} - 1} \right),$where $\frac{U_{i}}{\delta i}{and}\frac{U_{j}}{\delta j}$ are the meanedge field stress levels of two neighbouring sections, wherein at leastone section is formed by a non-cylindrical field-grading layer and$\frac{U_{i}}{\delta_{i}^{\prime}}{and}\frac{U_{j}}{\delta_{j}^{\prime}}$are the mean edge field stress levels of two corresponding neighbouringsections the corresponding condenser bushing with the cylindricalfield-grading layers.
 17. The electrical insulation system according toclaim 13, wherein the radial widths of the sections at their axial endsare substantially equal.
 18. The electrical insulation system accordingto claim 13, wherein the innermost and/or the outermost field-gradinglayer is cylindrical.
 19. The electrical insulation system according toclaim 13, wherein the capacitances of all the sections formed by thefield-grading layers are equal.
 20. The electrical insulation systemaccording to claim 13, wherein at least one edge of at least one of thefield-grading layers is bent outwards with respect to the axis.